The practice of audio technology is characterized by ever increasing use of multiple amplifiers and multiple loudspeakers, either in a monophonic mode where one or more primary sources such as microphones feed a single channel, or in the increasingly prevalent stereophonic mode employing two or more channels. In either mode it is increasingly common to feed multiple amplifiers from each channel and to feed multiple speakers from each amplifier, as found in a large variety of audio equipment such as stereo home music systems, automobile stereo systems which may utilize four or more amplifiers driving four or more speakers, multi-amplifier multi-speaker sound reinforcement systems for musical performance, auditorium sound systems and the like.
It is well known that multiple speakers in a system should always be operated in phase with each other for best results, and that otherwise, operating one (or more) speaker out of phase with the other(s) degrades both the efficiency and the fidelity of sound performance, particularly at low frequencies, due to cancellation effects. Correct speaker phasing is clearly important in multi-speaker monophonic systems, and is equally important in stereo systems: even though stereo systems are made capable of a high degree of interchannel isolation, in practice high degrees of program material separation are relatively infrequent overall since it has become customary in the artistic judgments of originatin and mixing stereo sound material to direct substantial portions of the audio source content to both channels in common, particularly at low frequencies where limited power-handling capabilities in reproduction are an important consideration. Thus the importance of correct speaker phasing in monophonic systems applies equally to stereo systems, and has become even further emphasized by the close identification of "stereo" with "high fidelity".
In audio practice involving such audio systems the need frequently arises to quickly and conclusively determine whether all of the amplifiers and speakers in a multi-speaker system are connected with correct polarity so that the speakers operate in phase with each other.
In new packaged audio equipment having internal speakers it is often assumed that, as originally furnished, the speakers would be properly connected in phase. However, where consumers hook up separate speakers, and even in packaged units which have been serviced or otherwise modified, there are many possibilities for unintentional loss of correct speaker phasing. The original phasing may have been reversed in the process of repairing or replacing speakers, speaker wiring, and/or amplifiers. In recognition of potential polarity errors, reputable manufacturers often provide polarity code markings on speaker terminals, and sometimes on amplifier output terminals; however such markings, while playing a useful role in polarity maintenance, are frequently misundestood or ignored, and in many instances are not readily accessable, being concealed within an enclosure.